U.S. Pat. No. 4,379,216 describes a method of making multilayer electrical-resistance-soldered tubing by first forming a flat copper-clad steel strip into a U-shape, then pressing this U-shaped workpiece into a tubular shape by engaging it with grooved rollers against a mandrel held stationarily inside the strip which is advanced downstream. Then an electrical current is applied to the strip by engaging it immediately downstream of the forming rollers with a contact shoe connected to one side of an alternating-current source and somewhat downstream therefrom by a pair of idler rollers connected to the other side of the source. The heating is adjusted by moving the contact shoe longitudinally along the workpiece, increasing or decreasing the resistance of the path through which the current flows. The voltage flowing in the workpiece heats it sufficiently to fuse the copper. The hot workpiece then passes through a cooling chamber where the copper resolidifies, soldering together the multilayer tube. The result is a tube of excellent strength with virtually no possibility of leakage due to the surface bonding of the several layers.
German patent 886,945 of B. Quarnstrom of 5 Oct. 1953 describes another such system which uses three sets of rollers to form a heating zone. The middle set of rollers is connected to one side of a single-phase alternating-current source, like that of the Weiss reference, and the two flanking roller sets are connected to the other side of the source.
German patent 2,828,960 of H. Dietzel describes a similar such system using single-phase alternating current in addition to inductive heating. In Konduktive Erwarmung von Metallen (1989; Rheinisch-Westfalisches Electrizitatitswerk AG; Essen, Germany; pp. 15ff) the uses of alternating and direct current in such heating applications are discussed. This system is only applicable to wires or workpieces of solid cross section, not to tubes.
Such systems can produce good-quality pipe, but in general the ease of control and the throughput could be improved.